Ankylosing spondylitis (AS) is a chronic inflammatory disease primarily affecting the spine and sacroiliac joints. The diagnosis of AS typically involves a combination of clinical evaluation, medical history assessment, imaging studies, and laboratory tests. Here are the common methods used to diagnose AS:

1. Medical History and Physical Examination: A doctor will ask about your symptoms, medical history, and perform a physical examination. They will look for signs such as limited spinal mobility, pain in the lower back or buttocks, stiffness, and other associated symptoms.

2. Imaging Studies:

   • X-rays: Plain X-rays of the pelvis and spine can reveal characteristic changes associated with AS, such as fusion of the vertebrae, sacroiliitis (inflammation of the sacroiliac joints), and bamboo spine appearance (ossification of spinal ligaments).
   • MRI (Magnetic Resonance Imaging): MRI scans are often used to detect early signs of inflammation in the sacroiliac joints and spine, especially when X-rays are inconclusive or to assess disease activity.

3. Laboratory Tests:

   • HLA-B27 Test: Human leukocyte antigen B27 (HLA-B27) is a genetic marker strongly associated with an increased risk of developing AS. However, it's important to note that not everyone with AS tests positive for HLA-B27, and its presence doesn't confirm the diagnosis.
   • Blood Tests: These may be conducted to measure markers of inflammation, such as erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP). While elevated levels of these markers can indicate inflammation, they are not specific to AS and can be present in other conditions.

4. Assessment of Symptoms and Criteria: The Assessment of Spondyloarthritis International Society (ASAS) has established classification criteria for diagnosing axial spondyloarthritis, which includes AS. The criteria consider various factors such as symptoms, age of onset, duration, and imaging findings.

It's important to note that diagnosing Ankylosing spondylitis can sometimes be challenging, as the disease may have a slow onset and can mimic other conditions. Consulting with a rheumatologist or a healthcare professional experienced in diagnosing and treating Ankylosing spondylitis is crucial for an accurate diagnosis.